It is the primary purpose of this project to study some of the pertinent factors which influence cell differentiation and malignant transformation, using techniques and approaches which range from the microscopic to the molecular level. Particular emphasis is given to those systems in which murine RNA tumor viruses or their precursors may be the transforming agent. A variety of mouse model systems are used, including plasma cell tumors, mammary tumors, and neuroblastomas. Current projects include: 1) characterization of the properties of intracisternal A particles; 2) relationship between type C viruses and solid tumors other than sarcomas; 3) correlation of cell ultrastructure with immunoglobulin secretion in Abelson virus-induced plasma cell tumors and lymphosarcomas; 4) effects of interferon on assembly of murine oncornaviruses.